default value in asp.net server control

Question

I have a problem with the default value attribute.

When I add my control to page at design mode, default value does not work. This is my code:

[DefaultProperty("Text")]
[ToolboxData("<{0}:KHTLabel runat=server key=dfd></{0}:KHTLabel>")]
public class KHTLabel : Label ,IKHTBaseControl
{
    [Bindable(true)]
    [Category("Appearance")]
    [DefaultValue("KHT")]
    [Localizable(true)]
    public string Key
    {
        get
        {
            String s = (String)ViewState["Key"];
            return ((s == null) ? String.Empty : s);
        }

        set
        {
            ViewState["Key"] = value;
        }
    }

    protected override void RenderContents(HtmlTextWriter writer)
    {......

But, at design mode when I add a control from the toolbox, the key does not exist

<cc1:KHTLabel ID="KHTLabel1" runat="server"></cc1:KHTLabel>

Answer 1

That's not what the [DefaultValue] attribute does, I'm afraid. What it does it allows the Visual Studio designer (specifically the "Properties" grid to determine what to show by default and therefore how to know to show the value as bold when it's different from the default.

It's upto you to have something in your code that holds the value "KHT" as the default value. There's a bit of relevant detail in this 2008 blog posting of mine.

The following code is fairly rudimentary, and I haven't been able to verify it compiles, but it should give you some idea how you could handle "forcing" the value of your DefaultValueAttributes into the ViewState:

private string GetDefaultAttributeValueForProperty(string propertyName)
{
    var attributesForProperty = (from prop in typeof(KHTLabel).GetProperties()
                 where prop.Name == propertyName
                 select System.Attribute.GetCustomAttributes(prop)).First();
    var defaultValueAttribute = (from attr in attributesForProperty
                 where attr.GetType() == typeof(DefaultValueAttribute)
                 select ((DefaultValueAttribute)attr).Value).FirstOrDefault();

    return Convert.ToString(defaultValueAttribute);
}
public KHTLabel()
{
    ViewState["Key"] = GetDefaultAttributeValueForProperty("Key");
}

Answer 2

The DefaultValueAttribute is not used to set the value for the property. It is used by the serializer to determine wheather it should serialize the value or not. You need to set your default value for the property in the constructor (or in OnInit). Using the DefaultValueAttribute makes the serialized data smaller if the property-value matches the DefaultValueAttribute value.